And I don't want any cop-out answers like "Oh these guys are pacifists cause they make others do the fighting for them", I mean a genuine faction of people with power, who reject war, even when it's convenient for them to do war, and even when going to war would be considered the right thing to do by the masses, out of a genuine principle of pacifism. How do you make those people seem like a convincing faction?
Look into the nomadic peoples of arabia who gained power through trade. They aren't a perfect example because they did engage in violence but this could easily be adapted. Petra is also a very cool city you could look into, it was heavily fortified by its natural surroundings and therefore easily defensible in a way that minimizes harm.
I think your best option is a highly religious sect of nomadic traders for a number a reasons. The religion gives a source of their non-violent philosophy, trade gives them economic power with the least possible violence, and being nomadic makes it difficult to take this power from them through violent means.
Self Defense would be acceptable right? It would fit with the religious order thing if they were highly trained in some non lethal combat.
It's up to OP really but yes imo. Pure pacifism to the extent of accepting death is idealism
So I feel like you have 3 routes,
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most obvious and probably requires the least weird stuff is they limit access to themselves. Either they hide from possible aggressors, they make themselves mobile so as to flee from possible aggressors (à la ComradeSharkfucker's comment) or they prevent access to their home by possible aggressors (flying city, force fields, mountain village, undersea towns).
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They make harming them less valuable than leaving them alone. Say they have super advanced technology or magic and a desire to help others but refuse to help anyone who harms them and their tech or magic doesn't work for other species. Or maybe when they're killed the person who killed them is mentally merged with them permanently, which has led to an aversion to violence in their society, where taking life is normally done to preserve someone's consciousness beyond death and only by a good friend who is willing to hear their thoughts for the rest of their life. Other cultures also don't want all their soldiers to get mind melded with a bunch of pacifists and leave them alone.
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Their conception of violence is different from others and they're willing to do something in self defence that others don't want to experience but which they don't think of as harmful (teleported to another continent, or dimension [a safe one], "infected" with super empathy, time-warped back into a baby [you're now going to live another 37 years how can you say I've harmed you?]).
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My first thought for a pacifist faction is in science fiction where it's some post scarcity group that is aloof to the Galaxy, above it all, etc. As for making them compelling I'm not sure. Maybe sub factions within that want to become more interventionist pushing back against their isolation.
Larry Niven's Known Space books have the Man-Kzin wars, where humans were pacifist and unarmed when they entered the conflict. At least initially, humans would use ship engines as makeshift weapons based on the theory of "any sufficiently powerful form of propulsion can also be a weapon."
